1. Field of the Invention
The present invention relates to a noise suppressing apparatus, and in particular to an apparatus which is used for transmitting, accumulating, encoding, and recognizing a voice (speech), detects a soundless section of an input signal including a surrounding noise (background noise) to estimate characteristics of the surrounding noise, performs a signal processing according to the estimated character, and reduces or suppresses a noise.
2. Description of the Related Art
In the prior art noise suppressing (reducing) apparatus, a spectrum subtraction method for reducing a surrounding noise or the like included in a collected voice signal to emphasize voice components has been adopted in application of a voice transmission or a voice recognition for a cellular phone.
In such a spectrum subtraction method, as disclosed in the Japanese Patent Application Laid-open Nos. 4-340599 and 7-306695, a sound presence/absence is determined, a soundless section (section with only a noise) is cut out, and the character of the voice is estimated by using a signal of the soundless section.
This will be described referring to the attached figures. A noise reduction device 1, as shown in FIG. 6, is composed of a sound presence/absence determiner 11 for determining a sound presence section and a sound absence section of an input signal, a noise spectrum estimating portion 12 for inputting the input signal and calculating an estimated noise spectrum according to a determined result by the sound presence/absence determiner 11, and a spectrum subtractor 13 for subtracting the estimated noise spectrum calculated at the noise spectrum estimating portion 12 from the input signal to output a signal in which a noise is suppressed.
Among these portions, the sound presence/absence determiner 11 compares a frame power nfpow of an input signal s1 with a threshold value thr_pow to obtain a determined value as the following equation:
                              determined          ⁢                                          ⁢          value                =                  {                                                                      0                  :                                                                              sound                  ⁢                                                                          ⁢                  absence                  ⁢                                                                          ⁢                                      (                                          nfpow                      <                      thr_pow                                        )                                                                                                                        1                  :                                                                              sound                  ⁢                                                                          ⁢                  presence                  ⁢                                                                          ⁢                                      (                                          nfpow                      ≥                      thr_pow                                        )                                                                                                          Eq        .                                  ⁢                  (          1          )                    
Also, the noise spectrum estimating portion 12 executes the operation shown in FIG. 7 in accordance with the determined value from the sound presence/absence determiner 11 indicated by the above Eq.(1).
In FIG. 7, the estimated noise spectrum is not calculated if the determined result of the sound presence/absence determiner 11 indicates “sound presence”, so that the estimated noise spectrum calculated by the preceding frames is used. Only when it is recognized that the determined result of the sound presence/absence determiner 11 indicates “sound absence” (at step S11), an input signal transformation to a frequency range is performed (at step S12) having f1 [w] and f2 [w] respectively for a real part of the spectrum and an imaginary part by an FFT (Fast Fourier Transform) calculation of an NT point. It is to be noted that “w” is supposed to be a variable indicating a frequency.
As a result, a spectrum amplitude f3 [ ] of the input signal is given by the following equation:f3[w]=√{square root over (f1[w]*f1[w]+f2[w]*f2[w])}  Eq.(2)
A noise estimation buffer f3buf [ ] [ ] (supposed to perform f3num frame accumulation) is updated as given by the following equation (at step S13):
                                                                                          f                  ⁢                                    ⁢                  3                  ⁢                                                            buf                      ⁡                                              [                        frm                        ]                                                              ⁡                                          [                      w                      ]                                                                      =                                                      f3buf                    ⁡                                          [                                              frm                        -                        1                                            ]                                                        ⁡                                      [                    w                    ]                                                                                                                                                                f3buf                    ⁡                                          [                      1                      ]                                                        ⁡                                      [                    w                    ]                                                  =                                  f3                  ⁡                                      [                    w                    ]                                                                                      }                            Eq        .                                  ⁢                  (          3          )                    
Then, the above-mentioned noise estimation buffer is averaged to obtain an estimated noise spectrum f3est [w] as given by the following equation:
                              f3est          ⁡                      [            w            ]                          =                              1            f3num                    ⁢                                    ∑                              frm                =                1                            f3num                        ⁢                                                  ⁢                                          f3buf                ⁡                                  [                  k                  ]                                            ⁡                              [                w                ]                                                                        Eq        .                                  ⁢                  (          4          )                    
The estimated noise spectrum f3est [w] thus obtained is provided to the spectrum subtractor 13 together with the input signal, for the spectrum subtraction.
The arrangement of the spectrum subtractor 13 is shown in FIG. 8, in which the input signal is converted into a signal of the frequency range at an FFT calculator 111, and the real part of the spectrum f1 [w], the imaginary part f2 [w], and the spectrum amplitude f3 [w] are obtained as described above.
The estimated noise spectrum f3est [w] given by the above-mentioned Eq.(4) is provided to a subtractor 112 to perform the subtraction.
At the subtractor 112, a noise reducing coefficient g1 [w] is firstly obtained by the following equation:
                              g1          ⁡                      [            w            ]                          =                                            MAX              ⁡                              (                                  0.0                  ,                                                                                    f3                        ⁡                                                  [                          w                          ]                                                                    *                                              f3                        ⁡                                                  [                          w                          ]                                                                                      -                                                                  f3est                        ⁡                                                  [                          w                          ]                                                                    *                                              f3est                        ⁡                                                  [                          w                          ]                                                                                                                    )                                                                    f3                ⁡                                  [                  w                  ]                                            *                              f3                ⁡                                  [                  w                  ]                                                                                        Eq        .                                  ⁢                  (          5          )                    
This coefficient is obtained by normalizing a difference (0 or more) between the power of the spectrum amplitude f3 [w] and the power of the estimated noise spectrum f3est [w] with the power of the spectrum amplitude f3 [w].
By using this coefficient g1, a real part f4 [w] and an imaginary part f5 [w] of the spectrum after the subtraction at the subtractor 112 will be calculated as given by the following equations:
                                                                                          f4                  ⁡                                      [                    w                    ]                                                  =                                                      f1                    ⁡                                          [                      w                      ]                                                        *                                      g1                    ⁡                                          [                      w                      ]                                                                                                                                                                f5                  ⁡                                      [                    w                    ]                                                  =                                                      f2                    ⁡                                          [                      w                      ]                                                        *                                      g1                    ⁡                                          [                      w                      ]                                                                                                          }                            Eq        .                                  ⁢                  (          6          )                    
An inverse FFT (Inverse Fast Fourier Transform) is performed to the real part f4 [w] and the imaginary part f5 [w] of the spectrum outputted from the subtractor 112 at a calculator 113, and then a signal (after noise reduction) s2 [n] is outputted
In addition to an embodiment of a noise reduction processing in a frequency range as mentioned above, it is also made possible in a time range. For example, the input signal is divided into a plurality of bandwidths by a bandwidth division filter and an estimated noise power for each bandwidth is obtained, whereby a suppressing processing has only to be performed so that the power may have the estimated noise power subtracted from the input power for each bandwidth at the spectrum subtraction.
In such a prior art noise reduction device, it is disadvantageous that the sound presence/absence can not be accurately determined when a signal noise ratio (SNR) is extremely bad, so that a spectrum estimation is performed in the sound presence section, thereby suppressing sound components.
In the Japanese Patent Application Laid-open No. 9-18291, such a technology is disclosed that the signal noise ratio is estimated, and an adaptive rate (step size) of an adaptive filter is controlled by the estimated value, thereby suppressing the noise.
However, in this Japanese Patent Application Laid-open No. 9-18291, it is disadvantageous that a single microphone is provided respectively for the input signal and a reference noise for controlling the adaptive filter, and two microphones in total are required, so that the hardware is enlarged and the cost is high.